PROJECT SUMMARY/ABSTRACT Nonunion is defined as the permanent failure of a bone to heal, where surgical intervention is required to achieve healing [1]. There are two types of nonunion, hypertrophic and atrophic, and at the 2018 Intl. Society of Fracture Repair (ISFR), a need for new models of atrophic nonunion was identified. Animal models of atrophic nonunion have been around as early as 1999, providing an in vivo representation of the bone's inability to heal, resulting in nonunion [1, 2]. However, the majority of the animal models used to replicate nonunion involve osteotomy, periosteal stripping, bone marrow removal, devascularization, or the creation of a critical-sized defect. While all approaches result in nonunion, such invasive methods are not representative of many clinical nonunions where osseous regeneration has been arrested by a disturbance of metabolic pathways [3, 4]. The ?failure of biology? seen in atrophic nonunion should, instead, be modeled closely to those seen clinically with a reduced/absent callus. Thus, there remains a need for the development of a more relevant, pre-clinical nonunion model to test therapeutic interventions. In a previous study by Jilka et al. [5], 3.6Col1a1-tk (Col1-tk) mice were developed in which proliferating osteoblast lineage cells can be ablated through exposure to the nucleoside analog ganciclovir (GCV). In preliminary studies in our lab, we have observed that ablation of proliferating osteoblasts in the Col1-tk mice causes a failure to form a callus. We posit that a more comprehensive assessment of the impaired healing in the Col1-tk mice may lead to its establishment as a useful atrophic nonunion model. Further, in atrophic nonunion, the bone healing has become stagnant and the addition of a biological agent, such as a bone graft, is often necessary to induce regeneration. We propose that a tissue-engineered scaffold-mediated gene delivery of a bone marrow mesenchymal stem cell (MSC) population overexpressing the osteogenic transgene bone morphogenetic protein-2 (BMP-2) will induce osteogenesis at the site of nonunion [6-10]. This research is significant to not only establish a clinically relevant model of atrophic nonunion, but also explore a novel rescue technique to restore healing, following the NIAMS mission of treating musculoskeletal injury and disease.